The present invention relates to an optical disc on and from which data is recorded and reproduced and to a disc drive apparatus for writing and reading data into and from such an optical disc and, in particular, to an optical disc in which data can be additionally written on a read only type optical disc and to a drive apparatus for writing and reading data into and from such an optical disc. As an external memory which can store a large amount of data, attention is paid to an optical disc drive apparatus using a read-only type optical disc (hereinafter, referred to as an R/O disc) in which data is recorded on a disc-shaped plastic material in the form of concave and convex pits of the submicron order and the data is reproduced by irradiating a laser beam having a diameter of about 1 .mu.m.
The R/O disc is formed in the following manner. The data modulated concave and convex pits of the submicron order are formed on a plastic resin made of polycarbonate or the like having a thickness of 1.2 mm. A reflecting layer made of aluminum or the like is evaporation deposited on the surface of the plastic resin and thereafter, a protecting layer is coated on the reflecting layer. Although the R/O disc has a diameter of 12 cm and a memory capacity of hundreds of megabytes, additional data cannot be recorded.
Various uses and applications of such an R/O disc are considered due to various advantages such that its memory capacity is large, random access can be performed, a number of duplicate discs can be easily produced, and the cost is low. For example, the R/O disc can be used as a disc in which the dictionary or font patterns for use in data base or word processors are stored, or as a disc in which programs or operating manually of computers are stored.
However, the foregoing R/O disc is of the read-only type and is manufactured in a particular factory. Therefore, it is impossible for the user to use the disc by adding his preferred extra data to the dictionary or font patterns of the data base or word processor which were supplied in the R/O disc. In addition, patch cannot be added in order to correct bug in the program. Therefore, the R/O disc has a problem such that its application field in computers is limited.